The present invention relates to a rotation detecting device for detecting a rotation of a detection subject using a magnetic force.
Patent Reference 1 has disclosed a conventional rotation detecting device (a magnetic sensor for detecting a rotation). The conventional rotation detecting device disclosed in Patent Reference 1 includes a detection element, in which a detection coil is wound around a wire-shaped magnetic element capable of generating a large Barkhausen jump. In the conventional rotation detecting device, a plurality of permanent magnets having polarities alternately changed is arranged with a constant interval on a drum-shaped substrate having a rotational center axis to be connected to a detection object.
Patent Reference 1: Japanese Patent Publication No. 2001-194182
In the conventional rotation detecting device disclosed in Patent Reference 1, when the drum-shaped substrate is rotated, the permanent magnets sequentially pass through near the wire-shaped magnetic element, so that an alternate magnetic field is applied to the wire-shaped magnetic element. As a result, an orientation of magnetization of the wire-shaped magnetic element is sequentially changed, so that a pulse signal representing the rotational state of the drum-shaped substrate is output from the detection coil.
In the conventional rotation detecting device disclosed in Patent Reference 1, the wire-shaped magnetic element extends in a direction in parallel to the rotational center axis of the drum-shaped substrate. Accordingly, it is difficult to reduce a size of the conventional rotation detecting device in a direction along the rotational center axis.
Patent Reference 2 has disclosed another conventional rotation detecting device (a rotation sensor). The conventional rotation detecting device disclosed in Patent Reference 2 includes a sensor coil, in which a coil wire is wound around an iron core formed of an amorphous magnetic member having a large Barkhausen effect. In the conventional rotation detecting device disclosed in Patent Reference 2, a permanent magnet is attached to a rotational board to be rotated in a specific rotational direction. The sensor coil is arranged such that an axial direction of the iron core is in parallel to a tangential direction of the specific rotational direction of the rotational board. Accordingly, it is possible to reduce a size of the conventional rotation detecting device disclosed in Patent Reference 2 in the direction along the rotational axis, as opposed to the conventional rotation detecting device disclosed in Patent Reference 1.
Patent Reference 2: Japanese Patent Publication No. 2000-161989
In the conventional rotation detecting device disclosed in Patent Reference 2, the sensor coil (the magnetic field detecting portion) is arranged such that a longitudinal direction of the iron core (the magnetic element) is in parallel to the tangential direction of the rotational direction of the rotational board with the permanent magnet attached thereto. However, the conventional rotation detecting device disclosed in Patent Reference 2 has the following problems.
In the conventional rotation detecting device disclosed in Patent Reference 2, when the rotational board is rotated, and the permanent magnet with the N polarity approaches one end portion of the sensor coil, the magnetic field generated with the permanent magnet passes through the iron core from the one end portion to the other end portion of the sensor coil. Accordingly, the iron coil is magnetized in one orientation. Further, when the rotational board is rotated, and the permanent magnet with the N polarity approaches the other end portion of the sensor coil, the magnetic field generated with the permanent magnet passes through the iron core from the other end portion to the one end portion of the sensor coil. Accordingly, the iron coil is magnetized in an orientation opposite to the one orientation described above. As a result, the pulse signal is output from the coil wire of the sensor coil according to the change in the orientation of the magnetization of the iron core.
As described above, in the conventional rotation detecting device disclosed in Patent Reference 2, if the orientation of the magnetization of the iron core is changed only when the permanent magnet approaches the one end portion or the other end portion of the sensor coil while the rotational board is rotating, it is possible to accurately detect the rational state of the rotational board. However, the orientation of the magnetization of the iron core may be changed in other occasion.
For example, when the rotational board is rotated, and the permanent magnet approaches a center portion of the sensor coil in a longitudinal direction thereof, the magnetization of the iron core become an unstable state, thereby changing the orientation of the magnetization of the iron core. However, the orientation of the magnetization of the iron core is not changed always when the permanent magnet approaches the center portion of the sensor coil in the longitudinal direction thereof. Rather, the orientation of the magnetization of the iron core may be or may be not changed. Further, it is difficult to predict whether the orientation of the magnetization of the iron core is changed or not.
In the conventional rotation detecting device disclosed in Patent Reference 2, it is presumed that the orientation of the magnetic field applied to the iron core by the permanent magnet is different in the range from the center portion to the one end portion of the iron core from the range from the center portion to the other end portion of the iron core. Therefore, it is difficult to predict whether the orientation of the magnetization of the iron core is changed or not when the permanent magnet approaches the center portion of the sensor coil in the longitudinal direction thereof.
More specifically, when the orientation of the magnetic field applied to the iron core is different in the range from the center portion to the one end portion of the iron core from the range from the center portion to the other end portion of the iron core, the orientation of the magnetization in the iron core is partially varied. Accordingly, the pulse signal output from the coil wire has a low output level.
Further, it is difficult to consistently generate the large Barkhausen effect, so that the output level of the pulse signal tends to be fluctuated. When the pulse signal has the low output level and tends to be fluctuated, it is difficult to accurately detect the pulse signal with a detection circuit at a later stage. As a result, it is difficult to accurately detect the change in the orientation of the magnetization in the iron core. Therefore, when the orientation of the magnetization is changed unpredictably, it is difficult to accurately detect the rotational state of the rotational board.
In view of the problems of the conventional rotation detecting device described above, an object of the invention is to provide a rotation detecting device capable of solving the problems. According to the rotation detecting device of the present invention, it is possible to prevent an unpredictable change in an orientation of magnetization of a magnet element constituting a magnetic field detecting portion. Accordingly, it is possible to accurately detect a rotation of a detection subject. Further, it is possible to reduce a size of the rotation detecting device in a rotational axis direction thereof.
Further objects and advantages of the invention will be apparent from the following description of the invention.